1. Field of the Invention
The present invention relates to titanium oxide and barium titanate powders, methods of manufacturing the same, dielectric ceramics and multilayer ceramic capacitors, and particularly to an improved method for preparing a highly crystalline fine barium titanate powder.
2. Description of the Related Art
Sintering a powder material mainly containing barium titanate results in a dielectric ceramic. The dielectric ceramic is used, for example, for forming dielectric ceramic layers included in multilayer ceramic capacitors.
For miniaturizing the multilayer ceramic capacitors and giving them high capacitance, it is effective to form thinner dielectric ceramic layers. In order to form thin dielectric ceramic layers, the barium titanate powder used for the dielectric ceramic layers needs to be finer.
Hydrothermal synthesis and hydrolysis have been suggested and put to practical use to readily obtain fine barium titanate powders, but these methods increase the cost of manufacturing the barium titanate powders. Accordingly, solid-phase reaction has been traditionally used for manufacturing barium titanate powders.
In the solid-phase reaction, starting materials, for example, a barium carbonate powder and a titanium oxide powder are mixed, pulverized with a medium, and then are calcined. For manufacturing finer barium titanate powder by solid-phase reaction, it is effective to pulverize titanium oxide powder or to use a much finer titanium oxide powder.
Unfortunately, while barium titanate is synthesized at temperatures of 1000 to 1200xc2x0 C., titanium oxide is sintered at about 800xc2x0 C. Hence, titanium oxide starts to be sintered, that is, grains grow, before the barium titanate starts to be synthesized. As a result, even when the fine titanium oxide powder is used, it cannot be effective enough to obtain a fine barium titanate powder.
Accordingly, it is an object of the present invention to provide a method of manufacturing a fine barium titanate powder by solid-phase reaction.
Another object of the present invention is to provide a titanium oxide capable of being advantageously used in the method of manufacturing the barium titanate powder and a method of manufacturing the titanium oxide powder.
Still another object of the present invention is to provide a dielectric ceramic prepared by sintering the barium titanate powder manufactured by the method of the present invention and a multilayer ceramic capacitor comprising the dielectric ceramic.
To this end, according to one aspect of the present invention, there is provided a titanium oxide powder comprising titanium oxide particles and a barium compound on the surfaces of the particles. Specifically, the titanium oxide powder has a layer mainly containing a barium compound with a predetermined thickness on the surfaces of the particles.
Preferably, the barium content of the titanium oxide powder is in the range of about 0.001 to 0.1 mol relative to 1 mol of titanium.
Another aspect of the present invention is directed to a method of manufacturing the titanium oxide powder described above.
The method comprises mixing a titanium oxide powder, a solvent and a barium-containing material soluble in the solvent to prepare a titanium oxide slurry. The solvent is removed from the slurry and the solvent-free mixture is heated so that a barium compound is present on the surfaces of the titanium oxide powder particles.
The barium compound stabilized on the surfaces of the particles in the heating step has a thickness of about 3 to 30 nm. The barium compound is in any one of three states depending on the heating temperature and the kind of barium-containing material.
The barium compound on the surfaces of the particles may be amorphous. A relatively low heating temperature of about 150xc2x0 C. or less facilitates creating this state.
The barium compound may in an amorphous state mainly containing BaCO3 on the surfaces of the particles. A medium heating temperature of about 150 to 600xc2x0 C. facilitates creating this state.
At least a part of the barium compound is a reaction product with titanium oxide at the surfaces of the particles. A relatively high heating temperature of about 600xc2x0 C. or more facilitates creating this state.
Another aspect of the present invention is directed to a method of manufacturing a barium titanate powder.
The method of manufacturing a barium titanate powder comprises mixing a titanium oxide powder comprising titanium oxide particles having a barium compound present on the surfaces of the particles and a powder containing barium to prepare a powdered mixture. Also, the powdered mixture is calcined.
Preferably, the barium content of the titanium oxide powder is in the range of about 0.001 to 0.1 mol relative to 1 mol of titanium in the method of manufacturing the barium titanate powder.
Preferably, the titanium oxide powder has a specific surface area of about 5 m2/g or more, and more preferably of about 10 m2/g or more in the method of manufacturing the barium titanate powder.
Another aspect of the present invention is directed to a dielectric ceramic comprising a powder material mainly containing a barium titanate powder prepared by the manufacturing method described above. The material powder is formed into a predetermined shape and is sintered.
Another aspect of the present invention is directed to a multilayer ceramic capacitor. The multilayer ceramic capacitor comprises a plurality of dielectric ceramic layers comprising the above dielectric ceramic and a plurality of internal electrodes. The internal electrodes extend along predetermined interfaces between the dielectric ceramic layers, and two opposing internal electrodes separated by one of the dielectric ceramic layers define a capacitor.